The title "Captain Marvel" has been used by characters at many comic book publishers, including the DC hero also known as Shazam. Nevertheless, it's now mostly associated with characters at Marvel Comics, namely Carol Danvers. Once known as Ms. Marvel, Danvers is only one of the many Captain Marvels seen in the Marvel Universe.
She was notably preceded by a mentor in the form of Mar-Vell, Marvel's first Captain Marvel. His successors include his children and unrelated heroes. One of the latter was even his former "sidekick" who similarly assisted other characters. This had made the mantle that bears the publisher's name one of the most muddled in all of Marvel Comics.
7 Mar-Vell
Debuting in Marvel Super-Heroes #12, Marvel's first version of Captain Marvel—the alien Mar-Vell—was created by Stan Lee and Gene Colan. A member of the Kree race, he became stranded on Earth and thus learned to appreciate its people. Taking the human identity of Walter Lawson, he defended the planet against earthbound and interstellar threats, including his own people. A later development tied his molecules to Rick Jones, a former "sidekick" for The Hulk and Captain America. This made it so that they couldn't exist on the same plane simultaneously.
It was an obvious attempt to imitate the original Fawcett Comics Captain Marvel but to no avail. Mar-Vell was never a popular character, to the point where his death has remained permanent since 1982. Though The Death of Captain Marvel was a lauded storyline, Mar-Vell's initial legacy rarely made much of an impression. In fact, he's mainly known for preceding the more popular Carol Danvers, who began as his "partner" Ms. Marvel.
6 Monica Rambeau
Roger Stern and John Romita, Jr. created Monica Rambeau, who was based on actress Pam Grier and debuted in The Amazing Spider-Man Annual #16. Released after Mar-Vell's death, the book had Monica bombarded by extradimensional energy. The media dubbed her "Captain Marvel" when she used her newfound powers, with The Thing noting that the original hero to use the name had died anyway.
As Captain Marvel, Monica actually led the Avengers and became the hero that a generation of readers most associated with the name. After losing and subsequently regaining her powers, she took the new name Photon. This is her most popular alternative name, but she's also gone by Pulsar and Spectrum.
5 Genis-Vell
Genis-Vell was the son of Mar-Vell and a female Titanian Eternal named Elysius. Created by Ron Marz and Ron Lim, he debuted under the name Legacy in Silver Surfer Annual #6 before eventually taking the Captain Marvel mantle. His ongoing titles were the first monthly Captain Marvel books from Marvel Comics in years when they began in the 1990s.
Genis-Vell is one of the more popular versions of Marvel's Captain Marvel. He's also one of the most powerful due to the combination of his parents' DNA. Later, abandoned the Captain Marvel moniker and used Monica Rambeau's old name Photon. This led to a strong rivalry between them until she started going by Pulsar.
4 Rick Jones
Mostly operating as a now somewhat forgotten sidekick character, Rick Jones theoretically became Captain Marvel—twice. On two occasions, he was the human of the powerful hero. In the first situation, he switches places with Mar-Vell in either the normal reality or the Negative Zone whenever one of them strikes the Nega Bands together.
The same thing happened with Genis-Vell, Marvel's son, though the Microverse was the destination instead of the Negative Zone. When bonded with Genis, Rick Jones was something of an unwitting mentor, reversing his status during the Silver Age of Comics. During his adventures with Captain Marvel's son, Rick even gained the power of "cosmic awareness." However, this was comedically used to comment on the Captain Marvel comic's cancelation, and Jones has never displayed the ability or the actual powers of Captain Marvel.
3 Phylla-Vell
Peter David and Paul Azaceta created Phyla-Vell, and she debuted in Captain Marvel #16. She was the second child of Mar-Vell and was created after her brother Genis recreated the universe. Her time trying to use the Captain Marvel name was contentious with Genis, who at the time had also been driven to insanity. As with other heroes in this mantle, however, this wasn't the only superhero name that she took.
Phyla-Vell was also known as Quasar, using the quantum bands taken from the first Quasar to make the identity into her own. She later made a pact with the Dragon of the Moon, becoming the new avatar of Oblivion and going by the name Martyr. This supposedly happened after she had given up her quantum bands, but the objects seem to now be a permanent part of her. She's usually romantically involved with fellow cosmic hero Moondragon, and her and her brother's names are a play on phylum and genus.
2 Khn'nr
Khn'nr was one of the Skrulls who had been ritually sent into secret identity as part of the Secret Invasion storyline. In his case, he was turned into a genetic replica of Mar-Vell, to the point where the heroic new persona overtook him completely. He eventually learned the truth about his origins, but this didn't stop him from emulating the hero he was forged to be.
To this point, the "new Captain Marvel" fought against his fellow Skrulls to protect Earth. He also saw issues with developments on Earth such as the Superhuman Registration Act and some of its sanctioned "heroes." This led to his death down the road, but not before he passed the mantle to another cosmic character. His name was seemingly a reference to Conner Kent, the modern Superboy from DC Comics.
1 Noh-Varr
Created by Grant Morrison and J.G. Jones, Noh-Varr debuted in Marvel Boy #1. A Kree enhanced via insect DNA, Marvel Boy was a powerful and resourceful young hero who quickly came to gain fans. He developed an attraction to Kara Sofen (the second Ms. Marvel) and a rivalry with Carol Danvers. This happened after Noh-Varr took on the Captain Marvel name for himself, something that Danvers disagreed with.
Noh-Varr later became the heroic Protector, though he wasn't always devoted to Earthbound heroism. Now trying to be more of a hero while also rebuilding the alien Kree Empire. He's no longer Captain Marvel, with Carol Danvers having long since taken that name. Nevertheless, he's one of the more popular and unique characters at Marvel to use the title at the publisher, proving that the right powers and a heroic heart can make anyone into the Marvel Universe's premiere hero.